1. Field of the Invention
The present invention relates to a medical apparatus used during the performance of eye surgery. In particular, the present invention is directed towards an automatic surgical device for cutting the cornea of a patient's eye and creating a hinged flap of corneal tissue. More particularly, the automatic surgical device of this invention includes a cutting head assembly which is specifically structured to move across the patient's eye along a generally arcuate path, and further, is readily usable on both eyes of the patient.
2. Description of the Related Art
The eye works on a principle very similar to that of a camera and is illustrated generally in FIG. 1. The iris I, or colored portion of the eye about the pupil P, functions like a shutter to regulate the amount of light admitted to the interior of the eye. The cornea C or clear window of the eye, and the lens L, which is located behind the pupil, serve to focus the light rays from an object being viewed onto the retina R at the back of the eye. The retina then transmits the image of the object viewed to the brain via the optic nerve, O. Normally, these light rays will be focused exactly on the retina, see dashed lines in FIGS. 2 and 3, which permits the distant object to be seen distinctly and clearly. Deviations from the normal shape of the corneal surface however, produce errors of refraction in the visual process so that the eye becomes unable to focus the image of the distant object on the retina. As one example, illustrated in FIG. 2, hyperopia or "farsightedness" is an error of refraction in which the light rays from a distant object are brought to focus at a point behind the retina, as indicated by the solid lines. As another example, illustrated in FIG. 3, myopia or "nearsightedness" is an error of refraction in which the light rays from a distant object are brought to focus in front of the retina, as indicated by the solid lines, such that when the rays reach the retina, R, they become divergent, forming a circle of diffusion and consequently, a blurred image.
Until about twenty years ago, such refractive errors could only be treated with eyeglasses or contact lens, both of which have well known disadvantages for the user. As one example, a patient having a large degree of refractive error will commonly be prescribed to wear a very thick and cumbersome pair of glasses, which the patient should wear at all times to correct his/her extremely poor vision. As another example, contact lenses, which are designed to fit directly over the cornea, can be difficult to insert and remove, and in any event, must be carefully cleaned and cared for. Even then, contact lenses may at times irritate the eyes of those patients who can wear them.
Consequently, in the last several years, research has been directed to surgical operations to change the refractive condition of the eye. Several methods and special instruments have been designed for performing this kind of surgery. One such technique was keratomileusis, developed by Dr. Jose Barraquer of Colombia in 1949, which required a precise reshaping of the cornea. The goal of corneal reshaping is to modify the curvature of the cornea, i.e., either to flatten or increase its curvature depending on the patient's condition, with the desired result being that light rays passing through the cornea will then be refracted to converge directly onto the retina. Keratomileusis was extremely difficult to perform because it required cutting the cornea to separate and remove a thin layer or section of corneal tissue from a patient's eye, termed the corneal cap, precise lathing of it into a new shape, and then replacing it and suturing it back onto the patient's cornea.
Keratomileusis has been abandoned in recent years to eliminate the requirement of lathing the corneal tissue and suturing it back into place. Automated Lamellar Keratectomy (ALK) is another surgical technique which developed as an outgrowth of keratomileusis, wherein the eye is first numbed by a drop of anesthetic, and then a suction ring is placed on the eye to carefully position the cornea (termed "centration" in the art) for being cut by a very fine microsurgical instrument known as a microkeratome. The microkeratome is generally a blade carrying device that must be manually pushed or mechanically driven in a cutting path across the suction ring simultaneous with the motorized movement of the cutting element, which movement is transverse to the direction of the cutting path. For treating myopia pursuant to ALK procedures, the microkeratome is typically used to first cut into the cornea so as to raise a thin layer of the anterior cornea of between 100-200 microns in depth and about 7 millimeters in diameter. Next, the microkeratome is then used to make a second pass over the cornea to resect or remove a smaller part of the cornea, generally about 4 to 6 millimeters in diameter, which is then discarded. The anterior corneal cap which was cut away with the first pass of the microkeratome is then put back into its original position, without suturing, for healing to occur. The desired result of this procedure is that the cornea will have a new curvature because of the resected tissue, which provides a new refracting surface to correct the patient's original myopic condition. To correct hyperopia under ALK however, the microkeratome is typically used to make a single deep pass over the cornea. The cut layers are put back into their original position, without any removal of any other tissue. Because of the depth of the cut, the intraocular pressure within the eye causes a steepening of the cornea to again, provide a new refracting surface which hopefully will correct the patient's original hyperopic condition.
In very recent years, it has been learned that in using the microkeratome to cut and separate a thin layer of the cornea, termed the anterior corneal cap, it is highly undesirable to completely separate this cap from the rest of the cornea. First, the corneal cap has been lost in some instances which is calamitous because the anterior segment of the eye must then be completely reconstructed. Second, it is also now known that following the reshaping of the cornea, the corneal cap should be carefully and precisely aligned back into its original position on the cornea; failure to do so may result in astigmatism or some unbeknownst refractive error. Consequently, it is now understood that the microkeratome should not sever the cap from the eye but instead, should leave a portion connected or "hinged" to the eye, thereby forming a raised layer of corneal tissue hinged to the eye, known as a corneal flap F, illustrated in FIG. 4. A significant problem however, is that presently known microkeratome devices do not readily permit the formation of a corneal flap, F. Instead, known microkeratomes involve a degree of guesswork for determining where on the eye to stop cutting movement of the microkeratome across the cornea so as to form the corneal flap. Further, there are multiple issues that a surgeon needs to consider in corneal flap construction, the three most important factors being: flap thickness, flap size and hinge size.
Another advance has been made in more recent years in surgical procedures to correct refractive errors of the eye, namely, the introduction of laser procedures to accomplish the reshaping of the cornea. One such procedure, known as Laser Intrastromal Keratomileusis, (LASIK), is currently considered optimal because it allows sculpting of the cornea without damaging adjacent tissues. Moreover, with the aid of computers, the laser can be programmed by a surgeon to precisely control the amount of tissue removed, and significantly, to permit more options for the reshaping of the cornea. Under LASIK procedures, the eye is still typically positioned within a suction ring and a microkeratome is typically used to cut into the cornea so as to raise a thin layer of the cornea. As described, it is now preferred that a corneal flap be formed. Significantly, it has been determined that the corneal flap should have a depth of no less than 130 microns and no more than 160 microns to yield optimal results. It should be borne in mind that achieving this result during surgery requires an extremely precise instrument as one micron is a unit of length equal to one thousandth of a millimeter. During laser surgery, the flap of corneal tissue is then gently pushed aside to expose and permit reshaping of the cornea by the laser. Consequently, the microkeratome is less frequently used to reshape the cornea, as occurred under ALK procedures, but is still used to cut into and to raise a thin layer of corneal tissue. A significant problem however, is that presently known microkeratome devices do not offer the degree of precision currently needed to properly and consistently form a corneal flap, instead of a corneal cap, let alone a corneal flap having a dimension within the range of currently desirable depth and a vastly improved smooth cut. Further, it has been determined that a larger diameter of the eye should be presented, as much as 8 to 10 millimeters, for corneal reshaping by the laser. This is because the laser can now be used to re-shape the corneal surface about a perimeter of the eye rather than at the center, which is believed to result in more accurate correction of refractive errors. Doing so however, requires that a sufficiently large diameter of the eye be presented and exposed, which is not possible to achieve with known microkeratome devices. For example, known suction rings for positioning the eye during surgery would likely require a greatly expanded frame, and that that frame be located lower on and about the girth of the eyeball, in order to expose a greater portion thereof. Such an assembly would likely be very difficult to employ given the physical space limitations of the eye socket.
Finally, known microkeratome devices typically cut across the cornea in a linear direction along a horizontal plane. That is, known microkeratome devices typically cut across the cornea in a direction starting from the side of the eye near the temple, proceeding horizontally across the face towards the nose. As a result, even if such microkeratomes were able to be effectively used to construct a corneal flap, let alone one of the currently desired more precise dimensions, the hinged portion of the corneal flap would be oriented at right angles to the natural blinking action of the patient, which is in the vertical plane. It is believed that it would be most optimal to construct a corneal flap having a hinged portion which is oriented to correspond with the natural blinking action of the patient in the vertical plane. It is however, believed that known microkeratomes cannot move linearly in a vertical plane because of the restrictions presented by the size of the eye socket formed by the cheek and brow bones of the human skull.
Thus, there is a need for an improved automated microkeratome which automatically and consistently permits the formation of a corneal flap, and which allows for even more precise construction of the corneal flap so as to result in a flap thickness of no less than 130 microns and no more than 160 microns, and a flap size between 8 and 10 millimeters in diameter. There is also a need for an improved automated microkeratome which more smoothly cuts across the cornea in forming the corneal flap so as to permit it to be precisely aligned back into its original position on the cornea following the reshaping of the cornea. Ideally, any such improved automated microkeratome will also permit construction of the corneal flap in such a way that the hinged portion of the flap will be oriented to correspond the natural blinking of the eye.